The invention relates to a marine structure that is adapted for being positioned on the bottom of a body of water, such as the sea bottom or ocean bottom.
The invention relates in particular to a marine structure that is designed for carrying out drilling operations in the marine bottom, such as for drilling boreholes into oil and/or gas containing subsurface formations. For this purpose, such structure includes a drilling platform and at least one conductor pipe at least extending between the platform and the bottom part of the structure.
The present invention further relates to a method of drilling a hole by means of such a marine structure.
Conductor pipes in marine structures are applied to form a communication between the wellhead on the platform and the hole that is being drilled in the marine bottom. These conductor pipes are of large diameter (say 30 inches) and are applied during spudding-in of the well as a means for guiding the drill bit and drill string down to the marine bottom, and as a means through which the drilling fluid is returned to the platform. In the later stages of the drilling operations, casing is suspended in the conductor pipe.
In drilling operations taking place in deep water it has been found that a close control should be carried out on the density of the mud that is being applied, in particular during the initial drilling period of the hole, that is the period prior to setting the first casing. The density of the mud should be kept sufficiently low to prevent the pressure exerted in the open hole by the weight of the return mud column to grow larger than the fracturing pressure of the formation around the open hole. Reduction of the density of the mud may be accomplished by reducing the solid content thereof. This, however, may influence the sealing properties of the mud adversely and should be avoided when drilling in formations with high permeability.
Object of the invention is a marine structure comprising means that allow the application of a relatively low mud pressure in the hole that is being drilled.